Angular Form validation using ControllerAs

I am currently facing the following problem:

I would like to validate my form input using the Angular ng-model directives, when using those together with $scope they work fine.

Now working with the Controller As syntax they fail to work.

This problem is poorly documented, the only help I could find is: http://ift.tt/1qAjNoS

Here is a small example of my code:

The html template gets called with Controller as vm

<form name="vm.signUpForm" ng-submit="vm.signup(vm.user)">

    <label for="name">Name</label>

      <input type="text"
                  class="form-control"
                  id="name"
                  name="name"
                  placeholder="Full name"
                  ng-model="vm.user.name"
                  ng-minlength="2" required />

    <div ng-show="vm.signUpForm.$submitted || vm.signUpForm.name.$touched">
        <span ng-show="vm.signUpForm.name.$error.required">Please fill in your name</span>
        <span ng-show="vm.signUpForm.name.$error.minlength">A minimum of 2 characters is required</span>
    </div>
...
</form>

Am I forced to use $scope for angular form validation ? Or did I miss something ?

Thanks in advance